Tin of 'Horlicks X-Ray Shadow Meal' barium sulphate compound by Horlicks Ltd., Slough, England, 1938-1950.

Bones show up easily on an X-ray but soft tissues such as guts and intestines hardly show at all. To improve the quality of the X-ray, what is known as a contrast medium is used. This is intended to improve the detail of these otherwise barely visible structures.

For example, after a patient drinks a solution of barium sulphate, the guts show up as an opaque white colour in subsequent X-rays. To make the liquid easier to drink, it was sometimes flavoured with the popular malted milk drink Horlicks. Barium sulphate is still used today to improve contrast in X-ray images.